A substrate processing apparatus such as a vertical type substrate processing apparatus is used for heat treatment of a substrate in a process of manufacturing a semiconductor device. A predetermined number of substrates (wafers) are arranged in a vertical direction and supported by a substrate retainer of the vertical the substrate processing apparatus. The substrate retainer is loaded into a process chamber of the vertical type substrate processing apparatus. Then, as a process gas is introduced into the process chamber with the substrate heated by a side heater which is provided outside the process chamber, substrate processing such as the heat treatment is performed.
The vertical type substrate processing apparatus may be include a surface type sub-heater for auxiliary heating disposed at a lower portion of a process chamber as well as a side heater. The surface type sub-heater reduces the time necessary for recovering and maintaining the temperature of the entire process chamber to shorten the processing time of the substrate.
On the other hand, when the diameter of the wafer is large, it is difficult to heat the center portion of the wafer. Therefore, it takes a long time to raise the temperature of the center portion of the lowermost wafer in the process chamber. Since the peripheral portion of the wafer is heated by both the side heater and the sub-heater with the conventional sub-heater, the temperature gradient profile of the lower portion of the process chamber during the raising of the temperature of the process chamber is different from the temperature gradient profile of the process chamber stabilized at a preset temperature, and the thermal velocity in the lower part of the process chamber stabilized at the preset temperature is different from the thermal velocity in the lower part of the process chamber without the sub-heater. Therefore, in the process of raising the temperature of the process chamber, the temperature distribution among a plurality of substrates and the temperature distribution at the surface of the substrate are not uniform. As a result, the recovery (temperature stabilization) may take longer.